Recently, technology for charging various devices such as mobile devices in a wireless manner has been developed. Wireless power transmission technology mainly uses magnetic induction type transmissions and self resonance type transmissions.
Magnetic induction type charging is used with a device and a charger relatively close to each other, generates electromagnetic waves by supplying alternating current (AC) power to a transmission coil, transmits an induction current having the same frequency as that of a receiving coil, coupled to the transfer coil, and performs charging with an induction current.
Self resonance type charging is undertaken using magnetic resonant coupling between a transmission unit and a reception device “charging target,” “target charging device” or “charging device” are also used). In the case of a transmission antenna, a magnetic field generated by AC power applied by a source coil is applied to a resonance coil of a transmission antenna by inductive coupling. Then, self-resonant coupling occurs between the resonance coil of the transmission antenna and the receiving antenna having the same resonant frequency, which in turn causes inductive coupling to a load coil and applies current to a load.
Self resonance type charging allows electrical power to be transmitted up to several meters, as compared to the magnetic induction type charging, allows multiple terminals in a single wireless charger body to be charged, and can perform charging, even if centers of transmission and reception coils are not exactly aligned with each other.
However, depending on the characteristics of the antenna, there may be a large variation according to the position of the antenna when multiple terminals are charged. That is, when reception devices are placed above an antenna of the wireless charger to be charged, a difference in charging speeds depending on position may occur, and thus, product reliability may be deteriorated.